Introduce ass_get_mass_userdata() and ass_digest_get_user_domain().

* ass_get_mass_userdata() - grabs a bunch of user data (user_login,
  user_nicename and user_email) in one DB query.  Cache results in
  $bp->ass->massdata for later reference.

* ass_digest_get_user_domain() - basically a copy of
  bp_core_get_user_domain() but references our $bp->ass->massdata array to
  avoid having to use bp_core_get_user_domain() in a foreach loop.